390 



REPTIL1A ORDER II.CHELONIA. 



the Cinosternidce and Ckelydriace, an additional series of small horny shields 

 is interposed between the marginal shields and those of the plastron. 

 Whereas in the single representative of the genus first mentioned, the plastron 

 is fully developed, in the other two genera it is cruciform, and connected 

 on each side with the upper shell only by a very narrow bridge. 



Far better known than the members of the last family are the large 

 Chelonians commonly termed snappers and alligator-terrapins, all of which 

 are now confined to the northern portion of America, one 

 Snappers and species ranging as far south as Guatemala and Ecuador. In 

 Alligator- their relatively large head and long scaly tails, these terra- 

 Terrapins. pins resemble the Platysternidve. They ditfer, however, in 

 Family the presence of long, rib-like processes, running backwards 

 ChdydridcK. from the nuchal bone of the carapace to underlie the mar- 

 ginals, and likewise by the imperfect bony roof to the lateral 

 regions of the hinder part of the skull. The vertebrae of the tail have, on 

 the other hand, the same structure, the articular cup being on the hinder 

 surface in the majority. The small, cross-shaped plastron is but very loosely 

 attached to the upper shell ; and it is not till a late period of life that tho 

 shell becomes fully ossified, large spaces long remaining in both the uppi r 

 and lower halves. The long tail bears on its upper surface a crest, and all 

 the toes, except the fifth hind one, are clawed. The front border of tho 



carapace is notched in order to 

 receive the ponderous head, 

 which cannot be withdrawn into 

 the shell. The largest member 

 of the family is Temminck's 

 snapper {Macroclemmys tem- 

 mtftdbi), which is generally to 

 be seen in the reptile-house at 

 the "Zoo," and ranges from 

 Missouri to Florida. The very 

 large head has the eyes lateral ; 

 and the tail is not so long as in 

 the members of the second 

 genus. In some specimens the shell measures twenty-four inches in length. 

 The two species of alligator- terrapins, or snapping turtles (Chelydra), have the 

 eyes directed upwards, and the tail exceeding the shell in length. The mem- 

 bers of both genera are common denizens of the fresh-waters of the United 

 States, where they are found in rivers, swamps, and ponds. In habits they 

 are very active, rushing on their prey with extraordinary speed, and feeding 

 entirely on animal food. The extraordinary power of their jaws (of which 

 the upper one is hooked) renders their bites very severe ; and bathers should 

 always keep a good look-out to see that none of these reptiles are in their 

 vicinity before venturing on a plunge. In captivity they are dull and sluggish 

 creatures. Several females lay their eggs in the same spot. 



Ttie excellence of the flesh of one species, and the commercial value of the 



tortoise-shell yielded by a second, render the turtles a family of special 



importance and interest. From all the families yet men- 



The Turtles. tioned the turtles are easily distinguished by the modification 



Family of their limbs into oar-like flippers, adapted solely for swim- 



Chelonidce. ming, and also by the heart-shaped form of the shell. There 



are never more than two claws on each limb, the head cannot 



Fig 5. TEMMINCK'S SNAPPER (Macroclemmys 

 tern inincki). 



